Silver chloride monocrystal doped with cadmium and low concentration of lead

ABSTRACT

The photographic properties of silver halide monocrystal particle-track detectors which contain cadmium are highly improved by an additional content of lead-II ions.

United States Patent 1191 1111 3,799,692 Haase et al. 1 1 Jan. 9, W73

[54] SILVER CHLORIDE MONOCRYSTAL [56] References Cited 3,047,392 7/1962Scott ..96/94 BF [75] Inventors grx figssz} ggfi zg 3,219,449 11/1965Saxe et al ..96/94 BF nus both of y 3,362,797 l/l968 Shaskolskaja..96/94 BF [73] Assignee: Agia-Gevaert Aktiengesellschaft, primary E i jTravis Brown Leverkusen, Germany Assistant ExaminerW0n H. Louie, Jr. 221Filed: 06. 29, 1970 A1wmey-C9nn9lly & Hutz [21] Appl. No.: 85,285

[57] ABSTRACT [30] Foreign Application Priority Data The photographicproperties of silver halide Nov 14 1969 German P 19 57 313 8 monocrystalparticle-track detectors which contain y cadmium are highly improved byan additional con- 52 us. c1. ..96/l08, 96/27 E, 96/94 BF, tem of252/408, 96/452 [51] Int. Cl. ..G03c 1/28 3 Claims, No Drawings [58]Field of Search ..96/94 BF, 108; 23/85, 87; 252/408 SILVER CHLORIDEMONOCRYSTAL DOPED WITH CADMIUM AND LOW CONCENTRATION OF LEAD The presentinvention relates to particle track detectors consisting of silverhalide monocrystals which are improved in their sensitivity and havetheir fogging reduced by the addition of certain doping agents.

In the investigation of atomic particles, especially in modern heavy ionphysics, solid state particle track detectors have achieved greatimportance for the detection of tracks of ionizing particles. Suchparticle track detectors must meet certain requirements, especially ifthey are to be used for quantitative measurements, and, in particular,the interaction of the particle under investigation with the solid mustdevelop in a characteristic and accurately reproducible manner.

The particle track must provide as much information as possible aboutthe particle. It must be able to be rapidly and easily interpreted.

Since the defects which an ionizing particle produces in the solid aresubmicroscopic in size, mechanism for amplifying the track must beavailable for purposes of photo-optical interpretation, e.g. forrendering the track visible. The defects produced in the solid by theionizing particles represent the latent image of the particle track,which is developed by the amplification mechanism. The more the trackdiscloses details characteristic of the particles, the better is thedetector.

Two amplification mechanisms have attained practical importance:

1. Selective etching of the solid along the particle track.

2. Deposition of a new phase along the track.

Etching has become important inter alia in the case of mica and someinorganic glasses and, in particular, organic high polymers. Theselective etching process along the particle track is mainly based onthe fact that bonds dissolved along the track considerably facilitatethe etching process. Numerous difficulties, however, arise in theetching process, which considerably restrict its utility.

The most serious disadvantage of the etching process is that valuabledetails are often lost, especially in the case of long particle tracks,because the etching medium must travel from the outside through thesolid along the track, and the etching channels available for this arevery narrow, with the result that the etching medium is often not ableto penetrate sufficiently deeply into the solid state detector. It is,therefore, in most cases not possible to amplify discontinuous particletracks by this method.

It is also known to detect tracks of ionizing particles in silverchloride monocrystals. In this type of detector, a new phase ispreferably deposited along the particle track. In the case of silverhalide mono-crystals, this new phase consists substantially of silver.

The silver chloride monocrystals are superior to the above-mentionedsolid state particle track detectors, in which the particle tracks mustbe amplified by an etching process, especially in that in silverchloride monocrystals the amplifying and development process can becarried out very simply and rapidly. The amplification process consistsin a uniform exposure of the crystal platelets, in which the particletrack was recorded, to high energy light, preferably UV light.

The development process can be explained as follows:

The exposure to light causes electron-defect electron pairs to beproduced in the crystals. The electrons are trapped along the particletrack in interchange with silver ions from the disturbed regions. Thetrack is in this way stabilised and then amplified. This process is, inprinciple, comparable to the elementary photographic process. Theoriginal track is the latent image of the track and the amplificationthen corresponds to the photographic development.

The disadvantage of these silver chloride monocrystal detectors was, inthe first instance, their inadequate reproducibility. This disadvantagewas overcome by using highly purified silver chloride for the productionof the detectors. It is known that such silver chloride is in itselfinsensitive and useless for the production of detectors, but by theaddition of certain substances, these silver chloride crystals can berendered sensitive to ionizing particles. This was achieved, e.g. by theaddition of cadmium or lead. In this connection the article by K.Breuer, G. Haase and E. Schopper in British Journal of Applied Physics,18 (1967) page 1824 et seq. and the publication by K. Breuer, E.Schopper, G. Haase and F. Zorgiebel in Phot. Korrespondenz 104 (1968)page 76 et seq.are to be noted. The silver chloride crystals doped asdescribed above are sufficiently sensitive for many purposes. They areadvantageous also in that they do not register 'y-rays, X-rays andelectrons, so that these rays do not produce an interfering background.

For more accurate quantitative measurements on tracks of ionizingparticles, the silver chloride monocrystals doped with cadmium alone,which were, as such, very promising, were still in need of improvementboth with regard to their sensitivity and especially with regard to thebackground, i.e. the signal-to-noiseratio. This background, whichinterferes: with the interpretation of the particle tracks, is duemainly to:

I. lattice defects which are already present in the crystals from thestart, i.e. even before the irradiation of the particles, and which cannever be completely avoided, especially displacements or small anglegain boundaries of general substructures which, rather like the latticedefects produced by irradiation of the particles, are decorated withsilver along the particle tracks in the amplification process;

2. silver particles which are statistically distributed in the crystaland which have been formed by photolysis during the amplificationprocess (print-out);

3. deposits which are formed in the course of production of the crystalsand which, owing to the limited solubility of cadmium in silver halide,occur especially in silver halide monocrystals doped with highconcentrations of cadmium, and which cause optical clouding which canconstitute a serious interference, especially in fairly thick crystals.

It is among the objects of the present invention to provide improvedsilver halide particle track detectors which have higher sensitivity tolight and a reduced background.

We now have found silver halide monocrystal detectors doped with cadmiumfor recording tracks of ionizing particles, which detectors in additioncontain lead ions in quantities of up to ppm as a second doping agent.

This small addition of lead ions in general lead-II ions substantiallysuppresses the interfering background while the sensitivity to lightremains the same, so that substantially better particle track imageswhich can be more reliably interpreted are obtained. It is to be assumedthat the small additional doping with lead causes the above-mentioneddevelopable defects which are already present in the monocrystal, suchas crystal shift, etc., to become insensitive to such an extent thatthey no longer form a background during the amplification process byexposure to UV light.

The effect of the lead addition according to the invention isparticularly unexpected in view of the fact that it was known that thesensitivity of silver halide crystals towardsionizing particles couldbeincreased by the addition of lead and that a relatively powerfulbackground which prevents quantitative measurements occurs also in thosedetectors which are doped with lead alone. It could, therefore, not bepredicted that suppression of the background could be achieved by theaddition of small amounts of lead to cadmiumdoped silver chloridemonocrystals.

By means of the particle track detectors which are doped in accordancewith the present invention, ion tracks can be recorded without aninterfering background. The concentration of cadmium may vary withinwide limits. It depends primarily on the nature of the ionizingparticles which are required to be detected with any given detector.Concentrations of about 50 ppm (parts per million) up to about 1 percentby weight of cadmium, based on the weight of the silver halide,preferably silver chloride, have generally been found to be sufficient.If the amount of cadmium added is small, only decay products and heavyions can be detected. In this way, an interfering background due to theeffect of light particles can be largely avoided. At cadmiumconcentrations of over 0.1 percent by weight, practically all theionizing particles, even the lighter ones, are recorded.

The required quantity of lead ions is 5 to 100 ppm, preferably 5 to ppm.

The particle track detectors according to the present invention may beused for determining particle data, for the investigation of particlereactions and nuclear fissions, the investigation of decay mechanismsincluding those of superheavy nuclei, the identification of isotopes ofhigh energy ions and the investigation of isotope compositions of solarradiation or of cosmic radiation to determine the sources of thisradiation. These detectors are especially suitable for recording tracksof heavy ions.

The track of ionizing particles can be amplified in the detectorsaccording to the present invention in the usual manner by uniformexposure to shortwave light, especially UV light. Extremely sharplydefined tracks can thus be obtained on a clear background.

In this respect, the detectors are superior to conventional photographicemulsions for recording nuclear tracks (nuclear track emulsions). Theseemulsion materials consist of a silver halide gelatine emulsion layerwhich has a high power of resolution, on a layer support. With thesephotographic emulsions, it is generally not possible to obtain suchsharp particle tracks as in the detectors according to the presentinvention. The photographic emulsions, moreover, generally have a morestrongly interfering background since they are also sensitive to'y-rays, X-rays and electrons.

In the detectors according to the present invention, tracks of ionizingparticles can be amplified along their whole length for practically anygiven length, even if the tracks are discontinuous, i.e. if between thecrystal regions of high interference, which are produced by the ionizingparticles running through them, there are crystal regions which areundisturbed or relatively little disturbed, in which the track isinterrupted. This follows inevitably from the nature of theamplification process since in the amplification process which occursinside the volume, electrons and silver ions are exchangeably depositedwherever lattice defects have been produced by the ionizing particlesrunning through the volume. In this respect, the detectors according tothe present invention are generally superior to detectors in which thetrack amplification is produced by an etching process. The etchingprocess starts at the surface of the detector, where the ionizingparticle has entered the crystal, and continues along the track of theparticle into the interior of the crystal, and fresh etching solutionmust be supplied along the channel already formed by the etchingprocess. In the case of discontinuous particle tracks, the etchingprocess is liable to stop at the end of a track section because theetching solution then cannot penetrate sufficiently rapidly-the adjacentundisturbed region of the crystal, so that the following sections of thetrack which are not continuous with the previous track can no longer beamplified. In the case of discontinuous particle tracks, a lessdisturbed or even undisturbed crystal region between two track sectionsmay occasionally be penetrated by the etching solution if the timeallowed for the etching solution to act is considerably increased. Inthat case, however, the etching solution also continues to act duringthis period in that portion of the track which was etched first andwhich has therefore been amplified, with the result that this firstsection of track becomes greatly increased in width and may acquire apronounced cone shape. This, however, seriously impairs thereproducibility of the track and the accuracy of the interpretation. Thedetectors according to the present invention are completely free fromsuch disadvantages.

The possibility provided by a silver halide monocrystal detectorsaccording to the present invention of producing an amplification ofuniform sharpness and high reproducibility even of those particle trackswhich start at some depth within the detector opens up fields ofapplication for these detectors in which other solid stateparticle-track detectors hitherto known could not be used with the sameassurance and accuracy. An example of this is the study of thedevelopment of the decay process with time. If amplification of theparticle tracks is carried out first at a point in time t, and then at apoint in time t,, it is possible to determine which tracks have beenadded during the time interval t, 1,, i.e. which new decay processeshave taken place inside the detector during the time interval t, t

The amplification process by uniform exposure of the detectors toshortwave light is characterized by its simplicity and freedom frominterference. After the recording of the particle track, the detectorsare not exposed to any liquids, so that any disturbances which might becaused by liquids are avoided. It is worth mentioning here by way ofcomparison, the sensitivity of the etching methods in this respect andthe swelling and distortion phenomena which occur in the conventionalphotographic processing of nuclear track emulsions.

EXAMPLE An aqueous solution of cadmium chloride (CdCl '2.5 H O p.a.) isadded in a pipette to pulverulent silver chloride which has a degree ofpurity of 99.999 percent, resulting in a silver chloride which has acadmium content of about 600 ppm. The mixture is dried in the pipette ina drying cupboard.

An aqueous solution of lead chloride (PbCl p.a.) is then added to thismixture in the same manner, resulting in a silver chloride which has theabove cadmium content and in addition a lead content of about ppm. Themixture is again dried in the pipette in a drying cupboard.

The silver chloride doped with cadmium and lead is melted in the pipetteand the melt is placed between two quartz glass platelets which areheated to about 550C and the distance between which is fixed at about200 p. by rods of quartz glass. On cooling, a polycrystalline silverchloride platelet doped with cadmium and lead is obtained.

The sandwich, consisting of the two quartz glass platelets with thesilver chloride platelet between them is placed in a horizontal quartzglass tube which, after being evacuated, is filled with nitrogen to apressure of 400 mm Hg. A tube furnace is then passed over the quartzglass tube at such a temperature and at such a rate that thepolycrystalline silver chloride platelet doped with cadmium and lead isconverted in a known manner, by a melting process, into a monocrystalwhich can be dissolved from the quartz platelets by dipping the sandwichinto water.

After irradiation of the ionizing particles which are to be investigatedand whose tracks are to be recorded, the silver chloride monocrystaldoped with cadmium and lead is uniformly exposed to a. Xenon highpressure lamp, a filter being interposed between the source of light andthe detector so that only a narrow range of wavelengths in the region of417 millimicrons comes into play. The intensity of the shortwave lightirradiating the specimen is about 10 quanta per cm per second. Theexposure time is about 20 to 30 minutes.

We claim:

1. In a silver chloride monocrystal for detecting the track of ionizingparticles and containing 20ppm to 1% by weight of cadmium as a dopingagent that increases its tracking sensitivity, the improvement accordingto which the crystal also contains, as a second doping agent thatreduces background interference with the track detection, lead-(II) ionsin a concentration of 5 ppm to ppm.

2. The combination of claim 1 in which the cadmium doping is in the formof cadmium-(II) ions present in a concentration of 100 ppm to 5000 ppm.

3. The combination of claim 1 in which the concentration of lead ions is5 to 20 ppm.

2. The combination of claim 1 in which the cadmium doping is in the formof cadmium-(II) ions present in a concentration of 100 ppm to 5000 ppm.3. The combination of claim 1 in which the concentration of lead ions is5 to 20 ppm.